Cancer is a leading cause of death in the United States and throughout the world. Cancer cells are often characterized by constitutive proliferative signals, defects in cell cycle checkpoints, as well as defects in apoptotic pathways. There is a great need for the development of new chemotherapeutic drugs that can block cell proliferation and enhance apoptosis of tumor cells.
Conventional therapeutic agents used to treat cancer include taxanes and vinca alkaloids, which target microtubules. Microtubules are an integral structural element of the mitotic spindle, which is responsible for the distribution of the duplicated sister chromatids to each of the daughter cells that result from cell division. Disruption of microtubules or interference with microtubule dynamics can inhibit cell division and induce apoptosis.
However, microtubules are also important structural elements in non-proliferative cells. For example, they are required for organelle and vesicle transport within the cell or along axons. Since microtubule-targeted drugs do not discriminate between these different structures, they can have undesirable side effects that limit usefulness and dosage. There is a need for chemotherapeutic agents with improved specificity to avoid side effects and improve efficacy.
Microtubules rely on two classes of motor proteins, the kinesins and dyneins, for their function. Kinesins are motor proteins that generate motion along microtubules. They are characterized by a conserved motor domain, which is approximately 320 amino acids in length. The motor domain binds and hydrolyses ATP as an energy source to drive directional movement of cellular cargo along microtubules and also contains the microtubule binding interface (Mandelkow and Mandelkow, Trends Cell Biol. 2002, 12:585-591).
Kinesins exhibit a high degree of functional diversity, and several kinesins are specifically required during mitosis and cell division. Different mitotic kinesins are involved in all aspects of mitosis, including the formation of a bipolar spindle, spindle dynamics, and chromosome movement. Thus, interference with the function of mitotic kinesins can disrupt normal mitosis and block cell division. Specifically, the mitotic kinesin KSP (also termed EG5), which is required for centrosome separation, was shown to have an essential function during mitosis. Cells in which KSP function is inhibited arrest in mitosis with unseparated centrosomes (Blangy et al., Cell 1995, 83:1159-1169). This leads to the formation of a monoastral array of microtubules, at the end of which the duplicated chromatids are attached in a rosette-like configuration. Further, this mitotic arrest leads to growth inhibition of tumor cells (Kaiser et al., J. Biol. Chem. 1999, 274:18925-18931). Inhibitors of KSP would be desirable for the treatment of proliferative diseases, such as cancer.
Kinesin inhibitors are known, and several molecules have recently been described in the literature. For example, adociasulfate-2 inhibits the microtubule-stimulated ATPase activity of several kinesins, including CENP-E (Sakowicz et al., Science 1998, 280:292-295). Rose Bengal lactone, another non-selective inhibitor, interferes with kinesin function by blocking the microtubule binding site (Hopkins et al, Biochemistry (2000) 39:2805-2814). Monastrol, a compound that has been isolated using a phenotypic screen, is a selective inhibitor of the KSP motor domain (Mayer et al., Science 1999, 286:971-974). Treatment of cells with monastrol arrests cells in mitosis with monopolar spindles.
KSP, as well as other mitotic kinesins, are attractive targets for the discovery of novel chemotherapeutics with anti-proliferative activity. There is a need for compounds useful in the inhibition of KSP, and in the treatment of proliferative diseases, such as cancer.